SAW Convolvers Using Focused Interdigital Transducers

A new SAW convolver which utilizes focused interdigital transducers for beamwidth compression has been demonstrated. Operating at a center frequency of 160 MHz, the device has a terminal convolution efficiency FT = -70 dBm. Here the results of theoretical design considerations together with experimental results from the prototypical device are presented. S I . INTRODUCTION URFACE acoustic wave convolvers have warranted much attention in recent years due to their ability t o perform sophisticated high-speed signal processing. In particular, relatively low-cost versions of these devices have been fabricated utilizing the inherent nonlinear elastic properties of piezoelectric substrates. The most widely used of these piezoelectric substrates has been lithium niobate. Although LiNb03 has a relatively high electromechanical coupling coefficient, its elastic nonlinearities are rather weak, necessitating the use of beamwidth compression techniques in order t o achieve reasonable convolver performance. Beamwidth compression is used in order to obtain highpower densities in the nonlinear interaction region without sacrificing the device bandwidth. Narrow interdigital transducers may be used to generate acoustic beams with highpower densities [ I ] , but these have the limitation that the impedance levels are very high. This makes it difficult to operate the transducers over a sufficiently large bandwidth, although recently narrow chirped transducers have been used with greater success [2] . Various forms of beamwidth compression have been utilized in the design of the elastic convolver [3] , [4] , [5] with the most developed designs incorporating either the multistrip coupler [3] or the parabolic horn compressor [4]. We report here on the use of the focused interdigital transducer as a means of beamwidth compression and report experimental results on a LiNbO, elastic convolver. The basic geometry of the focused transducer elastic convolver is shown in Fig. 1. The focused interdigital transducers and top-plate/waveguiding electrode are defined as a thin film metallization on the piezoelectric substrate. Electrical signals input into each of the focused transducers are converted into surface acoustic waves which counterpropagate each toward Manuscript received July 20, 1982;revised September 27, 1982. This work was supported by the Office of Naval Research under Contract N00014-75-C-0632. .l. R. Green was with Edward L. Ginzton Laboratory. W . W . Hansen Laboratories of Physics. Stanford University. Stanford, CA 94305. He is now with MIT Lincoln Laboratory. Lexington, MA 02173. G. S. Kino is with Edward L. Ginzton Laboratory, W. W. Hansen Laboratories of Physics, Stanford University, Stanford, CA 94305. ,--AV/V WAVEGUIDE METALLIZATION \ AND TOP-PLATE LECTRODE